The invention relates to a process for the preparation of liquid, storage-stable isocyanate mixtures exhibiting a low color number and containing carbodiimide (CD) and/or uretonimine (UI) groups. This invention also relates to the isocyanate mixtures obtained by this process, to their use for the preparation of blends with other isocyanates, for the preparation of prepolymers containing isocyanate groups, and for the preparation of polyurethane plastics, preferably polyurethane foams.
Isocyanate mixtures containing CD and/or UI groups can be prepared in a simple manner using the highly active catalysts from the phospholine series, in particular catalysts of the phospholine oxide series. Suitable processes are known and described in U.S. Pat. No. 2,853,473, EP-A-515 933 and U.S. Pat. No. 6,120,699.
The high catalytic activity of the phospholine catalysts, particularly the phospholine oxide catalysts, is on the one hand desirable in order to start up the carbodiimidization reaction under gentle temperature conditions. However, on the other hand, no process is known to date which ensures the effective termination of the phospholine catalysis or phospholine oxide catalysis without limitation. The carbodiimidized isocyanates tend to after-react, i.e. they release gas as a result of CO2-formation. This then leads to a build up of pressure in, for example, the storage tanks, especially when the temperature rises.
There have been numerous attempts to discover an effective means to terminate the phospholine catalysis. Suitable terminators are mentioned in, e.g., the patent specifications EP-A-515 933, EP-A-609 698 and U.S. Pat. No. 6,120,699. These include, e.g., acids, acid chlorides, chloroformates and silylated acids. Termination of the catalyst with acids, which can, for example, also be in the form of acid chlorides, is not sufficiently effective.
According to the disclosure of EP-A-515 933, CD/UI-containing isocyanate mixtures prepared by means of phospholine catalysis are terminated with at least an equimolar amount of, and preferably 1 to 2 times the molar amount of, trimethylsilyl trifluoromethanesulfonate (TMST), based on the catalyst employed. In practice, however, it has been found that CD/UI-containing isocyanates prepared in such a way are of only limited suitability for the preparation of prepolymers. Prepolymers are the reaction products of these CD/UI-containing isocyanates with polyols. The correspondingly prepared prepolymers, i.e. reaction products of polyols and the CD/UI-modified isocyanates, tend to release gas, which can lead to a build up of pressure in the transportation tanks or to foaming during handling of such products.
This problem can be avoided by employing the silylated acid used for terminating the phospholine catalyst, analogously to EP-A-515 933, in higher molar equivalents (e.g. 5:1-10:1, based on the catalyst employed). In practice, however, this results in the resultant CD/UI-modified isocyanates having a significantly poorer color number. This then also applies to the prepolymers prepared therefrom.
This also applies if the phospholine catalyst is terminated with acids of the trifluoromethanesulfonic acid type in accordance with U.S. Pat. No. 6,120,699. Prepolymers prepared therefrom also have a considerably increased color number.
The object of the present invention was to provide a process for the preparation of liquid, storage-stable isocyanate mixtures containing carbodiimide and/or uretonimine groups which does not have the deficiencies referred to above. It was also an object to prepare liquid, storage-stable isocyanate mixtures exhibiting low color numbers.